1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to an organic electrolytic solution securely adsorbed into the surface of lithium metal to make current distribution uniform and to increase the ionic conductivity of lithium ions during charging and discharging the invention also relates to a lithium secondary battery having improved lifetime characteristics that employs the organic electrolytic solution.
2. Description of the Related Art
Miniaturized, lightweight, thin and high-performance batteries for supplying power to portable electronic devices are in high demand, accompanying the technological development of portable electronic devices that have become miniaturized and lightweight, such as camcorders, portable communication devices or notebook computers. Research into such batteries therefore has been intensively on-going.
Lithium ion secondary batteries that are being widely used use carbon as negative electrode active materials and transition metal oxides (typically LiCoO2) as positive electrode active materials. In particular, carbon that is used as a negative electrode active material has a theoretical capacity of only 372 mAh/g, which is very low compared to a lithium metal having a capacity of 3860 mAh/g.
Unlike the lithium ion battery that uses a carbon material as the negative electrode material, a lithium metal battery uses a lithium metal instead of the carbon material as the negative electrode material. The use of a lithium metal as the negative electrode active material considerably reduces the volume and mass of the battery, which is the most significant advantage of lithium metal batteries. Research into secondary batteries therefore has pursued lithium metal batteries. However, such lithium metal batteries encounter several problems including rapid decrease in capacity due to repeated charge/discharge cycles, a change in volume during charging/discharging, instability, and the like. These problems all are caused by growth of lithium dendrites. Such problems associated with secondary batteries using a lithium metal as a negative electrode material make it impossible for the lithium metal batteries to be widely used, even if they have several advantages including the smallest density of 0.53 g/cm2, the highest potential difference of −3.045 V vs a standard hydrogen electrode (SHE), and the highest capacity per weight of 3860 mAh/g.
Various studies for preventing growth of lithium dendrites during charging are being actively carried out. There are two ways of stabilizing lithium: one is a physical method of suppressing growth of lithium dendrites by formation of a protective layer; and the other is a chemical method. Besenhard et al. (J. of Electroanal. Chem. 1976, 68, 1) discovered that the type of a lithium precipitate was greatly dependent upon the chemical composition and physical structure of a surface film. In other words, the physical formation of lithium dendrites results from a chemically uneven state of a surface film.
Yoshio et al. made approaches to increase the reversibility of a lithium negative electrode by controlling the surface state of a lithium metal such that an additive was added to a liquid electrolyte or lithium metal itself, as disclosed by in the 37th Battery Symposium in Japan, 1996. For example, an additive such as carbon dioxide, 2-methyl furan, magnesium iodide, benzene, pyridine, hydrofuran or a surfactant may be added to intentionally form a dense, thin and uniform surface film, thereby improving the surface state. These approaches have been attempted for the purpose of preventing formation of lithium dendrites by inducing uniform current distribution by forming a uniform, highly-conductive protective layer on the surface of a lithium metal.
Naoi et al. reported in J. of Electrochem. Soc., 147, 813 (2000)) that using the principle that the core of a helical ethylene oxide chain in polyethylene glycol dimethyl ether functions as a path of lithium ions during charging and discharging, a uniform protective layer could be formed for charging and discharging cycles by adsorbing polyethylene glycol dimethyl ether onto the surface of a lithium metal. Ishikawa et al., disclosed in J. of Electrochem. Soc., 473, 279 (2000) that charging/discharging efficiency could be increased by suppressing growth of lithium dendrites by formation of lithium alloys by adding aluminum iodide (AlI3) or magnesium iodide (MgI2) to an organic electrolyte.
However, these attempts still have a limitation in keeping a surface film at a uniform state over a repetition of charging and discharging cycles and passage of an immersion time. Also, in the case where each of the above-described attempts is made independently, a satisfactory cycle efficiency cannot be expected.
The description herein of various disadvantages and deleterious properties realized by certain known products, processes, and/or apparatus is by no means intended to limit the invention. Indeed, various aspects of the invention may include some of the known products, processes, and/or apparatus without suffering from the described disadvantages and deleterious properties.